Based on Patricia Highsmith's novel, "This Sweet Sickness" published in 1960.
Kelsey's birthday is 5th April - the same date as author of the screenplay Robert Bloch.
Dean Stockwell is 5' 7" and was born March 5th, 1936, his character David Kelsey's work file lists his height as 5' 9" and was born April 5th, 1936, exactly one month later.
Susan Oliver and Kathleen Nolan both went on to make guest star appearances (in separate episodes) of Magnum, P.I. (1980)